This invention relates to a process of making binderless briquets from fine-grained steelworks dusts, which contain metallic iron, which briquets are intended for use in smelting processes, wherein the steelworks dusts are heated and in a hot state are briquetted on a briquetting roll press and the hot briquets are air-cooled.
In steelmaking, highly fine-grained dusts are collected, e.g., in the electrostatic precipitators used to dedust the exhaust gases from converters and electric arc furnaces. Said dusts have high contents of Fe.sub.2 O.sub.3, FeO and metallic iron. The contents of CaO and MgO fluctuate greatly and may reach ranges from 3 to 30%.
Owing to the small particle size of the dusts collected in electrostatic precipitators, amounting to 60% below 0.045 mm, and the large surface area of said dusts, amounting to 7000 to 9300 cm.sup.2 /g, said dusts have a low bulk density of about 1000 kg/m.sup.3 and a specific gravity of about 5 g/cm.sup.3 and the metallic iron and FeO contained in said dusts have a high reactivity with oxygen or with oxygen-containing gases at temperatures above about 130.degree. C. The total iron content is from 60 to 75%, the FeO content is usually from 15 to 25% and the content of metallic iron may be as large as about 24%.
In dusts collected in evaporative coolers, the contents of CaO and MgO exhibit the same fluctuations as in the dusts collected in electrostatic precipitators but such dusts have a considerably larger particle size of about 90% between 0.045 and 1 mm. Their bulk density is about 3000 kg/m.sup.3 and their specific gravity about 6 g/cm.sup.3. Their total iron content is about 80 to 93%, their FeO content about 2 to 10% and their metallic iron content about 70 to 88%. The metallic iron and the FeO contents are oxidized with oxygen from the environment at about 350.degree. C.
Said dusts have such a high tendency to oxidize, that oxygen offered by the environment will result in an oxidation. An oxidation with decomposition of CO.sub.2 will also be caused by CO.sub.2 contained in flue gases. In an experiment conducted at a dust temperature of 650.degree. C. it has been found that flue gases effect an oxidation of about 9.4% Fe to FeO accompanied by a decomposition of CO.sub.2 into oxygen, CO and soot.
In view of these properties, said dusts cannot be stored and re-used in an untreated form.
German Patent Specification No. 3,223,203 teaches to briquet said dusts in that oxidizing gases are blown through the dusts at a temperature in excess of 200.degree. C. and the oxidation of part of the metallic iron results in a heating of the dusts to 450.degree. to 650.degree. C. Briquetting is effected on a briquetting roll press under a roll pressure in excess of 6 kN/cm roll width. The hot briquettes are cooled in an air stream to a temperature below 100.degree. C. The necessary reoxidation of the metallic iron offsets the work of reduction previously performed, which involved a large consumption of energy.